Light modulating material comprising polymer dispersed liquid crystals

ABSTRACT

A light modulating material is disclosed, comprising a solid, light transmissive, polymeric matrix having dispersed within said matrix droplets of a liquid crystalline composition, wherein said composition is a mixture of at least 2 compounds, at least one of said compounds having general formula (I), where n is 1 or 2, the fluorine substituent(s) may be in any of the available substitution positions and R 1  is C 1-12  alkynyl, hydrogen, R, RO or RCO where R is alkyl or perfluoroalkyl and at least one other of said compounds having general formula (II), where m is 0 or 1 and R 2  is hydrogen or C 1-12  alkyl or alkoxy.

This invention relates to novel light modulating materials comprising asolid, light transmissive polymeric matrix having dispersed thereindroplets of a liquid crystalline composition. The invention also relatesto novel compositions suitable for use in such materials.

Liquid crystalline materials are widely used in liquid crystal displayssuch as watches, calculators etc. Most displays of this type consist ofa thin film of a liquid crystalline composition sandwiched in a cellbetween two substrates, at least one of which is transparent, and havingtransparent electrodes on their inner surfaces. On applying a potentialdifference across the electrodes the alignment of the molecules of theliquid crystalline composition is altered, resulting in anelectro-optical effect in the material, which is exploited by thedisplay. Most often electro-optical effects in the nematic liquidcrystal phase are exploited in such displays. Examples of types of suchdisplays include the twisted nematic device, the Freedericksz effectdevice, cholesteric memory mode device, cholesteric to nematic phasechange effect device, dynamic scattering effect device, two frequencyswitching effect device and the "supertwist" effect device.

There are problems in constructing a large, e.g. several square metresarea, liquid crystal display device of the above type. For example,there are substantial engineering problems in simply constructing alarge cell with a constant substrate spacing of typically 1-10 μm overits entire area. Moreover, as liquid crystal materials are fluid theymay flow causing variations in the film thickness in the sandwich. Bothof these problems result in variations in the appearance or quality ofthe display over a large area. Consequently most present liquid crystaldisplays are at most a few square centimetres in area.

Recently a new type of liquid crystal display material has beendeveloped, in which droplets of a suitable liquid crystallinecomposition are dispersed in a solid, light transmissive polymericmatrix. In the art such materials are known generally as polymerdispersed liquid crystal (abbreviated "PDLC") materials and/or as noncurvilinear aligned phase (abbreviated "NCAP") materials because oftheir construction and physics respectively. Light is scattered fromsuch materials because of a mismatch between the refractive index of thedroplets and the matrix. The director of the liquid crystals within thedroplets has no preferred orientation but varies randomly from dropletto droplet in the absence of an electric field. The liquid crystal has apositive dielectric anisotropy and therefore aligns parallel to anapplied electric field. When no field is applied incident light isefficiently scattered because of the mismatch of the refractive index ofthe droplets and the matrix. When a field is applied the alignment ofthe liquid crystal results in the droplets having an effectiverefractive index closer to the ordinary refractive index of the liquidcrystal, n_(o), for light incident normal to the material. The matrixhas a refractive index, n_(p), essentially equal to n_(o) and thereforethe material becomes transparent to incident light.

Such materials may be made into large area display devices, such aswindows, privacy screens etc, without the problems mentioned above. Theconstruction of such a device is described below and illustrated in FIG.1 of the accompanying drawings. Further references to such materials anddevices are found in Mol. Cryst. Liq. Cryst. Inc. Nonlin. Opt. (1988)157 427-441, Liquid Crystals (1988) 3(11) 1543-1559, FR 2.139.537, U.S.Pat. No. 4,435,047 and U.S. Pat. No. 4,688,900.

To date, only a limited range of compounds have been used incompositions for PDLC materials. The references mentioned above onlyrefer to the use of benzylidineanilines, 4-alkyl or alkoxy4'-cyanobiphenyls or 4"-cyanoterphenyls, alkyl or alkoxycyanophenylcyclohexanes or cyanobiphenylcyclohexanes, and aromaticesters in these compositions.

There is clearly scope for development of new liquid crystalcompositions for PDLC materials. Although many types of compound andcomposition are known which are suitable for use in the small areadisplay devices discussed above, these compounds and compositions arenot necessarily suitable for use in PDLC materials in view of theirparticular requirements as described below.

The ordinary refractive index n_(o) of the composition should match orbe similar to the refractive index of the polymer matrix, so that in thepresence of an applied field the material becomes as transparent aspossible to incident light, preferably n_(o) being less than around 1.5.Further desirable characteristics of the liquid crystal composition area high birefringence, Δn, a high nematic to isotropic transitiontemperature T(N-I) and a low threshold voltage, Vth, for the potentialdifference of the field required to operate the material. In contrast =osmall scale liquid crystal displays viscosity, η, is less critical. Itis also desirable that PDEC compositions should use known andcommercially available liquid crystal compounds, to minimise researcheffort in preparing them.

The inventors have unexpectedly discovered that certain laterallyfluorinated 4-alkyl or alkoxy 4"-cyanoterphenyls may be mixed with otherknown liquid crystal compounds to provide exceptionally useful PDLCcompositions. Some such terphenyls are disclosed in Mol. Cryst. Liq.Cryst. (May 1988) 158B 209-240, and in PCT/GB89/00647 (priority 16 Jun.1988). Speculative disclosures of these terphenyls are also found in WO86/04081 and GB 2039937A. In none of these publications is there anysuggestion of suitability for use in PDLC compositions.

According to this invention a light modulating material is provided,comprising a solid, light transmissive polymeric matrix having dispersedwithin said matrix droplets of a liquid crystalline composition, whereinsaid composition is a mixture of at least two compounds, at least one ofsaid compounds having a general formula I: ##STR1## wherein n is 1 or 2,the fluoro substituent(s) may be in any of the available substitutionpositions and R¹ is C₁₋₁₂ alkynyl, hydrogen, R, RO or RCO where R isalkyl or perfluoroalkyl, and at least one other of said compounds havinga general formula II: ##STR2## wherein m is 0 or 1 and R² is hydrogen orC₁₋₁₂ alkyl or alkoxy.

Preferably at least one compound of formula I in the composition in thismaterial has a formula IA: ##STR3## where any one of W, X, Y or Z isfluorine, and where R^(A) is preferably alkyl or alkoxy. Of compounds offormula IA those where X is fluorine and/or where R^(A) is a n-propylare particularly preferred because of their high solubility in compoundsof formula II, and compounds of formula IA in which W, Y or Z isfluorine are also preferred because their use in the composition helpsto lower V_(th) and to increase T(N-I) and Δn. This is generally true ofall the PDLC materials and compositions discussed below.

In this material the composition may additionally include one or morecompounds of formula III: ##STR4## where R³ is C₁₋₁₂ alkyl or alkoxy and1 is 0 or 1. 1. The inclusion of compounds of formula III into thecomposition can lead to a high T(N-I), high Δn, and particularly a lown_(o), and this is generally true of all the PDLC materials andcompositions of the invention discussed below.

The invention also provides novel compositions suitable for use in PDLCmaterials of the type described above, which may be supplied alreadydispersed in the polymer matrix or alternatively may be suppliedseparately for subsequent dispersal in a polymer matrix.

One such composition of =his invention comprises one, two or morecompounds of formula I, preferably at least one being a compound offormula IA, mixed with one or more compounds of formula II at least oneof which has m being O and R² being ethyl, n-propyl, n-hexyl orhydrogen. The inclusion of one or more compounds of formula II with suchtermini is found to result in advantageous compositions having a lowerV_(th) and a higher Δn than for example if R² is n-pentyl, and this isgenerally true of all the PDLC materials and compositions of theinvention discussed herein.

Further compositions of this invention include one or more compounds offormula I having a formula IB: ##STR5## wherein R^(B) is alkyl oralkoxy, k is 1 or 2 provided that when k is 2 the fluorinesubstituent(s) may occupy any of the substitution positions but if k is1 then the fluorine substituent may only occupy the positions other thanX, Y or Z, together with one or more compounds of formula II.

Preferred compounds of formula IB are those of formulae IB1 to IB5below: ##STR6##

Especially preferred compounds of formula IB are those in which thefluorine substituent(s) point(s) towards the CN group, as this helps tolower V_(th) and to increase T(N-I) and Δn.

In formulae I and II above R^(A) and R^(B) preferably contain 1-6 carbonatoms, and are preferably alkyl.

The compositions discussed above may contain two or more compounds offormula I, and these may be one or more compounds of formula IA, or oneor more compounds of formula IB, or a mixture of one or more compoundsof formulae IA and IB.

The compositions discussed above may also advantageously additionallycontain one or more compounds having formula IV, V, VI or VII: ##STR7##wherein each of R⁴, R⁵, R⁵¹, R⁶ and R⁷ is independently C₁₋₁₂ alkyl oralkoxy; rings A and B are independently phenyl or cyclohexyl; p, q, rand s are each independently 1 or 2; and the fluorine substituent in VImay be in any of the available substitution positions.

Preferred compounds of formulae IV, VI and VII have formulae IVA, VIAand VIIA: ##STR8##

The inclusion of esters of formula IV can assist in achieving highT(N-I) mixtures having a high Δn. The inclusion of compounds of formulaVI and/or VII can assist in achieving a lower V_(th) and a high Δn. Theinclusion of terphenyls of formula V can assist in reducing viscosity,yet retaining high T(N-I). Inclusion of compounds of formula VI canraise T(N-I) without increasing the UV absorbance at longer wavelengths.

The composition may also include one or more pleochroic dyes.

Typically a composition as described herein contains 5-50 weight % ofterphenyls or formula I, especially 15-30 weight %. If the compositioncontains compounds of formula II having R² as ethyl, n-propyl, n-hexylor hydrogen then typically these are present at 10-50 weight %.Compounds of formula III may typically be present to about 5-40 weight%. Compounds of formulae IV, V, VI and VII may be present to a maximumof about 30 weight %. Pleochroic dyes may typically be present up toabout 1 weight %.

All of the compounds of formulae I to VII are known compounds which areeither available commercially or may be made via literature routes, andcompositions using them may be made using conventional liquid crystalmixing techniques. The selected compounds I to VII also enable a furtherdesirable characteristic, i.e. high solubility in monomers and lowsolubility in the resultant polymers, which assists droplet formation.

The polymeric matrix used in the materials or with the compositions ofthe invention may be those commonly used in this art. Suitable polymersinclude epoxy resins including thermosetting and thermoplastic epoxyresins, UV cured polymers, polyvinyl polymers such as polyvinyl alcohol,polyacrylates such as polymethylmethacrylate, polyurethanes, polyestersand polyarylalkenes such as polystyrene. Preferred polymers are theepoxy resin which is a mixture of epichlorohydrin and bisphenol A and acuring agent, the polyurethane which is a mixture based on toluenediisocyanate, polyether glycols, methylenebisisoorthochloroaniline andvarious polyols, polymethylmethacrylate and polyvinyl alcohol. Typicallymaterials of the invention contain 15-90% by weight of the composition,preferably 30-80%.

Materials according to the invention may be made using such polymers andthe compositions described using known techniques, such aspolymerization induced phase separation (PIPS), thermally induced phaseseparation (when the polymer is thermoplastic and can be melted) (TIPS),and solvent induced phase separation (SIPS). The use of these threebasis techniques is for example described and illustrated in Mol. Cryst.Liq. Cryst. Inc. Nonlit. Opt (1988) 157 427-441. This publication alsodescribes methods of controlling droplet size, which is typically aroundthe wavelength of the incident light, i.e. 1-10 μm, and density dropletscm⁻³).

Using other known techniques, the alignment of the director of theliquid crystal composition forming the droplets may be controlled as thematerial is formed, so that even when no electric field is applied tothe material the director is oriented in a desired direction by surfaceinteractions at the droplet-matrix interface. For example if thematerial is in the form of a sheet or film the directors may be orientedparallel to or normal to the plane of the sheet. This may be achievedfor example in known ways by straining the sheet, or applying anelectric field whilst the material is being formed.

The invention also provides an electro-optical light modulating devicecomprising a film of a material as described above typically 5-50 μmthick, having electrodes on opposite faces of the film by means of whichan electric field may be applied, at least one of said electrodes beinglight transmissive. A suitable material for these light transmissiveelectrodes is indium tin oxide. Conveniently the device may be mountedfor strength on a substrate or between two substrates at least one ofwhich is optically transmissive. Such substrates may for example be madeof glass or a transparent plastics material which may be flexible. Suchdevices may for example be used as windows, privacy screens, displaysignboards, traffic signs etc.

The invention will now be described by way of example only withreference to FIG. 1 which shows a cross section through a device of theinvention which incorporates a material of the invention.

EXAMPLE 1 Light Modulating Device

Referring now to FIG. 1 a device of the invention consists of a film ofa transparent solid polymeric matrix 1 having a refractive index n_(p).Dispersed within the matrix 1 are droplets 2 of a liquid crystallinecomposition. The matrix 1 with dispersed droplets 2 is sandwichedbetween two transparent indium tin oxide electrodes 3, 4 which arethemselves mounted on the surface of two transparent glass substrates 5,6.

Within the droplets 2 the director of the liquid crystal is oriented asa result of surface interactions between the liquid crystal and thepolymer matrix 1 of the droplet--matrix interface.

The liquid crystal composition has an ordinary refractive index n_(o)normal to the director and an extraordinary refractive index n_(e)parallel to the director. The extraordinary refracture index n_(e) issimilar to the refractive index n_(p) of the matrix, but there is amismatch between n_(o) and n_(p). When there is no electric field acrossthe film 1, as in FIG. 1A, the directors of the liquid crystal droplets2 are randomly oriented, and incident light I_(o) is scattered asscattered light I_(s) because of the mismatch between n_(o) and n_(p).Appearance of the device is therefore opaque milky white.

When an electric field is applied across the film 1 as in FIG. 1B byapplying a potential difference V between the electrodes 3, 4 thedirectors of the liquid crystal droplets align parallel to the directionof the field. The retractlye indices n_(p) and n_(o) match and thedevice becomes transparent, transmitting incident light I_(o) astransmitted light I_(T).

EXAMPLE 2 Compositions

Various compositions according to this invention are listed below. Theproportions of the compound are given in weight %. All alkyl and alkoxygroups shown are n-alkyl or n-alkoxy. T(C-N) is the solid to nematicliquid crystal melting point. ##STR9## T(N-I) 104.5° C. T(C-N) notfrozen

Δn 0.273

η20 62.9 c St ##STR10## T(N-I) 74° C. T(C-N) not frozen

Δn 0.257

η20 46.9 c St ##STR11## T(N-I) 104.6° C. Δn 0.238

η20 72.2

n_(e) 1.756

n_(o) 1.518

V_(th) 1.79 V ##STR12## T(N-I) 99.2° C. Δn 0.277 ##STR13## T(N-I) 62° C.Δn 0.246

V_(th) 1.26 V ##STR14## T(N-I) 73° C. Δn 0.257

η20 62 c St

V_(th) 1.30 V ##STR15## T(N-I) 101° C. Δn 0.265 ##STR16## T(N-I) 61.4°C. Δn 0.248

V_(th) 1.14 V

EXAMPLE 3 Light Modulating Materials

Material 1

Polyvinyl alcohol (PVA) (Vinol 205, Air Products) was purified bySoxhlet extraction with Methanol prior to use. To 15 g of a 20 weight %aqueous solution of the PVA was added 5 g of a liquid crystalComposition 9: ##STR17## T(N-I) 79° C. T(C-N) less than -20° C.

Δn 0.264

η20 68 c St

The PVA-Composition 9 mixture was emulsified using a laboratory stirrer,the droplet size then being measured using a Multisizer particle sizeanalyser (Coulter Industries) as having a mean volume diameter of 3.3 um(1-7 um diameter range). After degassing the emulsion to remove bubbles,the emulsion was coated as thin layers onto indium tin oxide coatedpolyester film. After allowing to dry for one hour another sheet of theoxide-coated film was laminated on top of the dried emulsion, then thelaminate was allowed to cure at 85° C. for a further 24 hours. Thethickness of the laminate was around 10 um.

The method was repeated using the same PVA and a second Composition, 10:##STR18## T(N-I) 93° C. T(C-I ) not frozen

Δn 0.278

η20 75.7 c St

which was found to be equally effective at forming such materials. Themethod was further repeated using Compositions 1-8 above, with similarsuccess.

Material 2

A thermosetting epoxy resin was formed by mixing Epon 828 (the reactionproduct of epichlorohydrin and bisphenol A) and Capcure 3-800 (atrifunctional mercaptan terminated liquid polymer), available from Shelland Miller Stephenson Company respectively. This liquid resin was madeinto a series of 2:1 by weight resin:composition mixtures withCompositions 1-10 above, which were then squeezed between glass slidescoated with indium tin oxide on the surfaces of the slides in contactwith the mixture to form a film about 25 um thick. The films were curedby heating between room temperature and 100° C.

Material 3

A thermoplastic epoxy resin was formed by mixing Epon 828 with oneequivalent of hexamine. This resin was made into a series of 2:1 byweight resin:composition mixtures with Compositions 1-10 above. Thismixture was cured at 70° C. for 12 hours to yield an opaque white solidwhich could be melted at 100° C. and formed into thermoplastic filmsbetween oxide coated slides as described above.

Material 4

A series of 1:1 by weight mixtures of powdered polymethylmethacrylateand Compositions 1-10 were prepared. Each mixture was dissolved inchloroform. The chloroform solution was poured over an oxide-coatedglass slide as described above and the chloroform was allowed toevaporate, leaving a thin film of the material. A second slide wasplaced oxide surface down onto the film and made to adhere by pressureand heat.

I claim:
 1. A light modulating material comprising a solid, lighttransmissive, polymeric matrix having dispersed within said matrixdroplets of a liquid crystalline composition, wherein said compositionis a mixture of at least 2 compounds, at least one of said compoundshaving the Formula I: ##STR19## wherein n is 1 or 2, the fluorinesubstituent or substituents may be in any of the available substitutionpositions and R¹ is C₁₋₁₂ alkynyl, hydrogen, R, RO or RCO where R isalkyl or perfluoroalkyl, and at least one other of said compounds havingthe Formula II: ##STR20## wherein m is 0 or 1 and R² is hydrogen orC₁₋₁₂ alkyl or alkoxy; provided that at least one of the compoundspresent given by formula I is given by the following formula: ##STR21##2. The material according to claim 1 wherein the compound of Formula Ihas the Formula IA: ##STR22## wherein R^(A) is alkyl or alkoxy and anyone of W, X, Y and Z is fluorine.
 3. The material according to claim 2,wherein the composition additionally includes one or more compounds ofthe Formula III: ##STR23## wherein R³ is C₁₋₁₂ alkyl or alkoxy and 1 is0 or
 1. 4. The composition according to claim 1 wherein at least one ofthe compounds of Formula I has the Formula IB: ##STR24## wherein R^(B)is alkyl or alkoxy, k is 1 or 2, provided that when k is 2 the fluorinesubstituent or substituents may occupy any of the substitutionpositions, but if k is 1, the fluorine substituent may only occupy thepositions other than X, Y and Z.
 5. The composition according to claim4, wherein at least one compound of Formula lB has the Formula lB1:##STR25##
 6. The composition according to claim 4, wherein at least onecompound of Formula IB has the Formula IB2: ##STR26##
 7. The compositionaccording to claim 4, wherein at least one compound of Formula IB hasthe Formula IB3: ##STR27##
 8. The composition according to claim 4,wherein at least one compound of Formula IB has the Formula IB4 or IB5:##STR28##
 9. The composition according to claim 1 which contains 2 ormore compounds of formula I.
 10. The composition according to claim 4wherein at least one compound of Formula I has a Formula IB and at leastone compound of Formula II in which m is 0 and R² an ethyl, n-propyl,n-hexyl or hydrogen.
 11. The composition according to claim 4, whereinthe composition additionally includes one or more compounds of theFormula III: ##STR29## wherein R³ is C₁₋₁₂ alkyl or alkoxy and 1 is 0or
 1. 12. The composition according to claim 1 wherein said compositionadditionally includes one or more compounds selected from Formulae IV,V, VI, VII: ##STR30## wherein each of R⁴, R⁵, R⁵¹, R⁶ and R⁷ isindependently C₁₋₁₂ alkyl or alkoxy; Rings A and B are independentlyphenyl or cyclohexyl; p, q, r, and s are each independent 1 or 2; andthe fluorine substituent in VI may be in any of the availablesubstitution positions.
 13. The composition according to claim 12wherein compounds of Formulae IV, VI and VII are selected from FormulaeIVA, VIA and VIIA respectively: ##STR31## where R⁴, R⁶ and R⁷ are asdefined in claim
 12. 14. The composition according to claim 10, whereinthe amount of compounds of Formula II, where R² is ethyl- n-propyl,nohexyl or hydrogen, is between 10-50 weight %.
 15. The compositionaccording to claim 10, wherein the polymeric matrix is selected from thegroup consisting of epoxy resins, polyvinyl polymers, polyacrylates,polyurethanes, polyesters and polyarylalkenes.
 16. The compositionaccording to claim 1 wherein said mixture contains a compound of theformula: ##STR32##
 17. An electro-optical light modulating devicecomprising a film of light modulating material according to claim 1 ofthickness between 5 μm and 50 μm, and electrodes on opposite faces ofthe film for application of an electric field, where at least one of thesaid electrodes is light transmissive.